Three-way conversion (TWC) catalysts have utility in a number of fields including the treatment of exhaust from internal combustion engines, such as automobile and other gasoline-fueled engines. Emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxides contaminants have been set by various governments and must be met, for example, by new automobiles. In order to meet such standards, exhaust articles containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. The TWC catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen.
Various regulatory agencies require that emission reduction equipment incorporated on a vehicle be continuously monitored by OBD systems. The function of these OBD systems is to report and set fault codes or alarm signals when the emission control devices no longer meet the mandated emission levels. One of the systems to be monitored is the TWC catalyst is used to simultaneously reduce the levels of carbon monoxide, nitrogen oxides, and hydrocarbons in the exhaust gases.
Internal combustion engines produce exhaust gases having compositions that oscillate over time between air/fuel (“A/F”) ratios that are slightly rich of stoichiometric and ratios that are slightly lean of stoichiometric. Ceria and other oxygen storage components are often included in automotive catalyst compositions to store oxygen when A/F ratios are lean of stoichiometric so that oxygen can be released when A/F ratios become rich to combust the unburned hydrocarbons and carbon monoxide. TWC catalysts are therefore characterized in one aspect with an oxygen storage capacity. As the TWC catalyst ages, however, its ability to store oxygen diminishes and the efficiency of the catalytic converter decreases. Based on this fact, current OBD systems in use today utilize linear air/fuel ratio sensors. Such sensors are typically referred to as exhaust gas oxygen sensors and are hereinafter referred to as “UEGO” if unheated and “HEGO” if heated. Typically, the UEGO will be located upstream of the catalyst and a HEGO and/or a UEGO will be located downstream of the catalyst to provide an estimate of a direct measurement of the oxygen storage capacity of the catalyst. Through calibration, this measurement of an estimate of the oxygen storage capacity of the catalyst can be related to the ability of the catalyst to convert the regulated exhaust gas emissions, i.e., the conversion efficiency of the catalyst. The catalyst deterioration can be therefore be monitored.
In particular, a typical method uses both UEGO and HEGO sensors that are electrochemical exhaust gas sensors, and their switching characteristics to ultimately monitor catalyst deterioration. The sensors detect whether the exhaust is rich or lean of stoichiometric. The method relies on measuring a ratio of the number of voltage level transitions (switches, e.g., across 0.5 volts) of two sensors, one placed upstream of the catalyst and one placed downstream of the catalyst. Contemporary catalytic converters have a significant oxygen storage capacity that dampens out the normal air/fuel cycling used in engine controller strategies. Therefore, the sensor placed upstream of the catalyst (measuring untreated exhaust from the engine) records a switch every time the exhaust gas moves from either a lean-to-rich or rich-to-lean state. The sensor mounted downstream of the catalyst, however, does not record a switch every time the upstream sensor switches, because the oxygen storage capacity of the catalyst acts as an integrator, smoothing out the A/F oscillations. As the catalyst deteriorates because of aging, the oxygen storage capacity of the catalyst decreases and therefore the downstream sensor records more switches. By monitoring the downstream sensor and upstream sensor switching transitions for a long period and ratioing the number of switching transitions, a parameter referred to as the switch ratio is obtained. This switch ratio is an indicator of the oxygen storage capacity of the catalyst. This switch ratio is then used as a diagnostic parameter for determining the pollutant conversion efficiency of the catalyst.
The amount of oxygen storage capacity that different vehicle original equipment manufacturers “OEM”s) require for various exhaust platforms is dependent on a number of factors including engine displacement, vehicle type, catalyst volume, catalyst location and engine management and can vary greatly according to the particulars of a vehicle and its associated exhaust platform. Alteration of the oxygen storage capacity of typical catalysts, however, can alter the catalysts' characteristics including their conversion efficiency. An increase or decrease in oxygen storage capacity to meet the OBD monitoring requirement can therefore be a burdensome process that can require multiple trials to finally arrive at the optimum oxygen storage capacity for a given exhaust system without sacrificing or altering catalytic efficiency or performance.
In addition to meeting on board diagnostic monitoring requirements, catalysts preferably meet requirements for efficiently using platinum group metals as catalytic agents due to the high cost of these metals. Strategies that have been used to optimize conversion efficiency with minimized platinum group metal usage include the use of zoned, gradient zoned and layered catalyst composites. Examples of the use of layers are disclosed in U.S. Pat. No. 5,597,771, while the use of zones is described in co-pending U.S. application Ser. No. 09/067,820 and WO 92/09848. Segregation of precious metals, such as platinum group metals, into layers and zones permits more control of the physical and chemical environment in which the individual precious metal components operate. For example, the catalytic activity of precious metal components are often more effective when in close proximity to certain promoters or other additives. In other instances to improve hydrocarbon combustion efficiency during cold starts, it is preferable to have high concentrations of certain catalytic agents such as palladium in frontal or upstream zones of the catalyst so that the exhaust gases contact these catalytic agents immediately and combustion of these pollutants can begin at low temperatures, as disclosed in U.S. Pat. No. 6,087,298.
The catalytic layers and zones are formed from washcoat compositions typically containing at least a refractory oxide support such as activated alumina and one or more platinum group metal (“PGM”) components such as platinum or palladium, rhodium, ruthenium and iridium. Other additives including promoters and washcoat stabilizers are often added. The washcoat compositions are deposited on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic honeycomb or a metal honey-comb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
Optimization of a catalyst so that it meets regulatory requirements for minimization of pollutants and durability, as well as an automobile manufacturer's requirements for precious metal usage and oxygen storage capacity for a given vehicle/exhaust platform, often requires extensive experimentation. The experimentation can include iterative reformulation of catalyst compositions and various layer or zone combinations followed by performance testing. It is often the case that the final optimized formulation of a one catalyst that successfully meets the requirements of one exhaust platform having a specific oxygen storage capacity requirement, cannot be used for a different exhaust platform having a different oxygen storage capacity requirement without extensive reformulation and performance testing. It would be preferable from the standpoint of both time and expense to have a more adaptable catalyst. Once having optimized a catalyst for catalyst performance and precious metal usage, it would be particularly desirable to be able to alter only the oxygen storage capacity of the catalyst without altering the performance requirements so that it can be used for different exhaust platforms, all having different OSC requirements.